Let's be generic, construct frameworks, describe the world in
an unified way...

No!, it is better to be specialized, design easy-to-use components,
offer plug-and-play objects...

Why not take advantage of the best of both worlds?

With Boost.Bimap, you can build associative containers in which both types
can be used as key. There is a library in Boost that already allows the creation
of this kind of container: Boost.MultiIndex. It offers great flexibility
and lets you construct almost any container that you could dream of. The
framework is very clean. You migh want to read this library's tutorial to
learn about the power that has been achieved.

But generality comes at a price: the interface that results might not be
the best for every specialization. People may end up wrapping a B.MI container
in its own class every time they want to use it as a bidirectional map. Boost.Bimap
takes advantage of the narrower scope to produce a better interface for bidirectional
maps [2]. There is no learning curve if you know how to use standard containers.
Great effort was put into mapping the naming scheme of the STL to Boost.Bimap.
The library is designed to match the common STL containers.

Boost.MultiIndex is, in fact, the core of the bimap container.

However, Boost.Bimap do not aim to tackle every problem with two indexed
types. There exist some problems that are better modelled with Boost.MultiIndex.

Problem I - An employee register

Store an ID and a name for an employee, with fast search on each
member.

This type of problem is better modelled as a database table, and Boost.MultiIndex is the preferred choice. It is possible
that other data will need to be indexed later.

Problem II - A partners container

Store the names of couples and be able to get the name of a person's
partner.

This problem is better modelled as a collection of relations, and Boost.Bimap fits nicely here.